An unidentified flying object was located near a nuclear power plant close to Russia’s second city, St. Petersburg, according to new reports.

An alert signal was sounded around the Leningrad nuclear power plant in the Russian town of Sosnovy Bor, local and state media reported on Wednesday. The object was moving at around 200 kilometers per hour (124 miles per hour) at an altitude of approximately 10,000 meters (33,000 feet), Russian sources reported.

The alert triggered a “special response status,” a military officer told Russian online newspaper, Lenta.ru. Reserve colonel Andrey Koshkin then suggested to state media there was “nothing unusual here, this is a regular situation.”

“It is possible that this is a weather balloon,” he added, which could be of “Russian or foreign origin.” However, Russian authorities have not yet provided an explanation for the unidentified object. The object was monitored by the Russian military, state news agency Tass reported.

The unknown object was “moving at the speed of wind,” Tass reported on Wednesday, citing an unnamed security source. “This means that it does not have engines, that is, it is not a drone, not an aircraft,” the source continued, adding there was “no reason” for the alert signal. However, “our air defense system is fully ready for action if necessary,” the source said, according to state media.

Tass then cited a Russian regional official as dismissing the reports. Newsweek has contacted Russia’s defense ministry for comment via email.

In late February, Russian authorities temporarily closed the airspace above St. Petersburg after unconfirmed reports of an unidentified object circulated. All flights were briefly suspended from Pulkovo, the city’s main airport.